Abstract
Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room temperature (T), the expected accompanying vapor and liquid H2O have not been found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy, and quenched the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150 °C. We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass during local, sub-micrometer-scale devitrification prior to sample collection. During MI heating experiments, the H2O is redissolved into the vapor in the bubble, where it remains after quenching, at least on the relatively short time scales of our observations. These results indicate that (1) a significant amount of H2O may be stored in the vapor bubble of bubble-bearing MI and (2) the composition of magmatic fluids directly exsolving from mafic melts at Mt. Somma-Vesuvius may contain up to 29 wt% H2O.
Acknowledgments
This work was supported by the 7/PON/ST/2012-4 Enerbiochem project to B. De Vivo. Partial funding for MI analysis was provided by PRIN2010PMKZX7 to C. Cannatelli. R. Esposito thanks E. Ammannito for discussions on peak fitting of the Raman spectra, and D. Moncada for help during heating/freezing experiments and Raman analysis at Virginia Tech. Funding was provided in part by the National Science Foundation (Grant Nos. EAR-1019770, Bodnar; EAR-1347987, Manning) and the Deep Carbon Observatory. We thank L. Danyushevsky for critical comments and help with heating experiments. We thank three anonymous reviewers for critical comments on an earlier version of this manuscript, A.J. Anderson, C. Bucholz, and M.L. Frezzotti for valuable comments that helped to improve this letter, and I. Swanson for editorial handling of this manuscript.
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